1. Field of the Invention
The present invention relates to a microconverter and a laminated magnetic-core inductor. More particularly, the present invention relates to techniques effectively applicable to a microminiature dc-dc converter to be mounted on a surface-mount circuit board (SM circuit board), generally called a microconverter, and a laminated magnetic-core inductor, i.e., a principal component of the microconverter, to be used for converting a supply voltage (electromotive force) of a built-in battery included in a portable information device, such as a portable telephone, into a predetermined working voltage to be applied to a circuit.
2. Description of the Related Art
A portable information device, such as a portable telephone, is provided with a dc-dc converter to convert the supply voltage (electromotive force) of a built-in battery into a working voltage to be applied to a circuit. The dc-dc converter includes an inductor, a main switching device and a control circuit. Technology relating to ICs (semiconductor integrated circuits) has made a remarkable progress in recent years, and the principal circuit elements including the main switching device and excluding the inductor have been replaced with an IC. Consequently, the principal components of the dc-dc converter are only an IC and an inductor. A dc-dc converter can be formed on a circuit board of an electronic apparatus by mounting the IC and the inductor on the circuit board.
The miniaturized dc-dc converter to be mounted on a circuit board is called a microconverter and is mounted together with other microdevices thereon included in a portable information device.
Although the IC having a comparatively high power capacity, i.e., one of the two principal components of the microconverter, is sufficiently miniaturized by fine processing techniques for fabricating semiconductor devices, it is difficult to miniaturize the inductor, particularly, the inductor included in a power unit, not to speak of forming the inductor in an IC. Thus, it may safely be said that the inductor is substantially the principal component of the microconverter, and the performance of the microconverter is greatly dependent on the performance of the inductor.
Generally, an inductor included in a power circuit, such as a dc-dc converter, is a transformer or a choke coil. The transformer and the choke coil are magnetic-core inductors each formed by winding a wire in a coil on a magnetic core. Therefore, the inductor is difficult, as compared with other electronic parts, to miniaturize and to form in a small thickness.